Manassas is the only school district in Northern Virginia that has decided not to send any of its students to the Fairfax County high school for science and technology next year.

The school, to be established in Annandale at the existing Jefferson High School, is expected to open in the fall of 1985. The Fairfax County School Board voted unanimously last week to open the school to students from seven other Northern Virginia jurisdictions, thus making it a state-designated "magnet" institution, one of four created in Virginia by Gov. Charles S. Robb.

Among the other jurisdictions outside Fairfax whose students are eligible to attend the regional high school, Loudoun County and the city of Falls Church signed the one-year contract with Fairfax 10 days before the Fairfax School Board approved the agreement Oct. 18.

Alexandria Superintendent Robert Peebles has not decided whether his school district will be involved in the regional high school during its first year, according to James Ekin, executive assistant for research and planning. Peebles is in the Soviet Union and will not submit a recommendation to the school board until after his return Oct. 24, Ekin said.

Peebles has said that his district might observe the school during its first year because "it makes sense sometimes not to plunge in." He also has said that the city's T.C. Williams High School has a reputation for having its own outstanding science program.

School boards in Prince William County and Manassas Park are expected to sign the contract in the next few weeks, according to the superintendents of both school districts.

In Arlington County, Superintendent Charles Nunley said he had recommended that the county participate in the regional science and tech high school, but that he could not predict how the school board would vote this week.

Manassas Superintendent F. Brent Sandidge said the School Board decided not to take any action last week on the one-year contract with Fairfax, which means the city merely will observe the school during its first year. Sandidge declined to give any reasons for the school board's action.

But he said, "It's new, for one thing, and we're not sure about the expense at this point."

Under the agreement approved by the Fairfax School Board, the other school districts will play only an advisory role in administering the school and will be responsible for the tuition and transportation costs of their students.

Some school officials in the seven jurisdictions outside of Fairfax have expressed doubts about the new school's drawing power, saying students are reluctant to give up their friends and football teams and put up with longer bus rides.